| 摘要 |
PURPOSE:To obtain an ftheta lens which can be corrected for a face collapse and is corrected for the chromatic aberration of the magnifying power by providing an ftheta function on the main scanning corresponding direction, and setting the starting point of deflection by a deflecting/reflecting face on the auxiliary scanning corresponding direction and a scanned face to the nearly conjugate relation geometrically and optically. CONSTITUTION:This ftheta lens has a three-group, three-piece structure. The first lens 51 has a concave sphere on the object side and a cylinder face with the negative refracting power in the auxiliary scanning corresponding direction on the image side. The second lens 52 has a concave sphere on the object side and a convex sphere on the image side. The third lens 53 has a flat face on the object side and a toric face with the positive refracting power in the auxiliary scanning corresponding direction stronger than the positive refracting power in the main scanning corresponding direction on the image side. The face collapse of a rotary polygon mirror is satisfactorily corrected, image face curves in main/auxiliary corresponding directions are satisfactorily corrected, and optical scanning can be realized by specifying the relation among the Abbe number vdj of the (j)th ((j) = 1-3) lens from the object side against the (d) line, focal distances fjM, fjS in the main/auxiliary scanning corresponding directions, and the synthetic focal distance fM in the main scanning corresponding direction of the whole system.
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